This invention relates to an arrangement for guiding the flue gases in a baking furnace of the ring section type for calcining carbon bodies, and to a procedure for operating such a furnace.
For baking carbon bodies for cells for the electrolytic reduction of alumina or for other electrometallurgical processes, special furnaces are used for the heat treatment (baking or calcining) of such carbon bodies.
The carbon bodies are made in the required shape from a mixture of crushed coke or anthracite and a binding agent which, for example, contains coils tar and pitch.
At room temperature, this mixture of coke and binder is stiff, but it becomes soft at temperatures over about 120.degree. C., giving off low-volatile components from the binder. When subjected to further heating over a period of time, to a maximum of 1,300.degree. C., the paste hardens, and its physical properties, such as electrical conductivity and resistence against oxidation, change.
Carbon bodies awaiting baking are usually referred to as "green carbons". These green carbons may weigh several tons and have a length of two meters or more. To prevent their becoming deformed when passing through a temperature range in which they become soft, special precautions have to be taken.
The green carbons are placed in deep pits in the furnace which is made of refractory bricks. The space between the carbons and the pit walls are filled with coke to support the carbons. Coke breeze also serves to protect the carbons against air combustion.
Several pits are built adjacent to one another, thereby forming a so-called section. In the walls between the pits there are channels, or ducts, for the flue gases. Heat is supplied to the carbons by passing the flue gases through these ducts.
The flue gases from one section pass, through ducts, to the adjacent section. In this manner, the flue gases can pass through several sections connected in series in a so-called firing zone. The usual fuels are oil or gas.
The flue gas vent and the burner manifold can be moved from section to section.
In a large ring furnace, there may well be two rows of sections built along side one another, thus forming parallel rows. At the end of a section row, the flue gas ducts are connected to the ducts in the parallel section row. In this way, the sections are joined together to form a ring. It is for this reason that such a furnace for baking carbon bodies is known as a ring section furnace.
In a ring section furnace there may be several firing zones in which the temperature is regulated according to a given program. The first sections in a firing zone have low temperature. These are followed by sections with higher temperature, while the final stage in a firing zone consists of those sections in which the carbons are cooled.
In a furnace of conventional design, each section is closed at the top by means of a section cover, and this has to be removed when green carbons are to be charged or baked carbons removed.
On account of the special properties of carbon bodies, it is necessary to avoid too large temperature gradients during baking, as these would result in cracks in the final product. Each section must therefore follow an exact time and temperature program. In the first part of the zone, the heating is up to 600.degree. C. by the heat in the flue gases from the last part of the firing zone. Later, in the temperature range from 600.degree. C. to the required top temperature (1,200.degree.-1,300.degree. C.) the heat must be supplied by the above-mentioned combustion of gas or oil.
In the cooling zone, the pit walls are cooled by air until the carbons can be removed without danger of oxidation.
Steps are taken to make the best possible use of the heat absorbed by the cooling air, by using this air for combustion.
The firing zone is moved by moving the oil or gas burners from one section to the next. The frequency of this operation is referred to as the heating cycle, and determines the capacity of the firing zone.
As already mentioned, it must also be possible to connect a gas exhaust system to a section to be connected to the firing zone. This is usually achieved by connecting a fan between this section and a pipe connection on an exhaust duct around the furnace. This exhaust duct is referred to as the flue ring main, and is kept under negative pressure by a main fan.
Before reaching the main fan, the flue gases usually pass through a scrubber which removes soot, tar vapour and other pollutants.
It is customary to differentiate between enclosed and open ring section furnaces. Enclosed ring section furnaces are usually built with vertical flue gas ducts in the pit walls. Several pits are built together, thus forming a section under a common section cover. With respect to the flue gases and the material to be calcined, the pits in a section are connected in parallel, while the sections are connected in series. There are horizontal flue gas ducts in the space below the section, while the gas flows unrestricted in the space between the section cover and the top of the pits. The flue gas ducts in the pit walls connect the spaces under the section cover with the space under the section.
With these vertical and horizontal ducts, this type of furnace has a larger total heat-transfer area than an open ring section furnace, in which the flue gas paths are restricted to the pit partition walls which are not interconnected inside each section.
Enclosed ring section furnaces have hitherto also been built with separate vertical ducts for firing, referred to as combustion chambers, to which the fuel is usually fed and combusted.
The usual practice has been that the flue gases flow vertically upwards in these combustion chambers, collect in the space under the section cover, and then flow vertically downwards in the gas ducts in the pit partition walls.